<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
	<style>
		.box{
			width: 400px;
			height: 600px;
			background: #222;
			margin: 0 auto;
			/*  */
			clip-path: polygon(0%  0%, 100% 0%, 50% 100%);
		}
	</style>
</head>
<body>
	<div class="box"></div>
</body>
</html>